1. Field of the Invention
The invention relates to a bandgap circuit, and more particularly to a bandgap circuit capable of operating under a fast power sequence.
2. Description of the Related Art
A bandgap circuit is a method of choice for developing a stable voltage reference and providing a bias current. The bandgap circuit is typically coupled with, or directly includes a start-up circuit. The main purpose of the start-up circuit is to start the bandgap circuit. The start-up circuit may ensure that the bandgap circuit operates within a valid operating point. As a supply power (VDDA) ramps up from zero volts to a final value, such as 18V, the bandgap circuit should reach its final value as well. Since it is possible for the bandgap circuit to remain at zero current and zero voltage, one of the start-up circuit's functions is to ensure that the bandgap circuit does not remain at zero current and zero voltage.
However, when the supply power is fast switched between it's ON and OFF states, for example, rapidly turned on and off in milliseconds, the fast power sequence causes the bandgap circuit to function abnormally. FIG. 1 is a schematic diagram showing how the abnormality of the bandgap circuit causes the band defect of a display device to happen. As shown in FIG. 1, the bandgap circuit 101 is a first stage of a circuit to provide a bias current. When the supply power VDDA is fast switched from an OFF state to an ON state as shown, the bias current cannot be successfully established (as the OFF state of the bias current shown in FIG. 1). Without the bias current, the receiver 102, such as a clock generator which receives the bias current to generate a clock signal, cannot generate the clock signal as well. Once the clock signal does not toggle, the logic circuit 103, which operates based on the clock signal, cannot be triggered. Therefore, the red, green or blue bands of a display device cannot be triggered correctly, and their states would be dominated by the corresponding initial states, causing the display band defect. When display band defect happens, the colors displayed by the display device will be incorrect due to the unknown initial states.
In order to solve the above-mentioned problem, a novel bandgap circuit capable of operating under a fast power sequence is highly required.